Oil and gas field chemicals

ABSTRACT

A method for conformance control of a reservoir comprising injecting into a zone of the reservoir an aqueous solution of a co-polymer comprising at least one ethylenically unsaturated polar monomer and at least one copolymerizable ethylenically unsaturated ester formed from a hydroxy compound of the formula ROH wherein R is a selected alkyl group, alkenyl group, cycloalkyl group, aryl group or such groups substituted with from 1 to 3 hydroxyl, ether or thio ether groups or a heterocyclic or selected heterocyclic alkylene group and at least one hetereatom selected from oxygen, nitrogen and sulfur and a selected alkenoic or aralkenoic carboxylic acid or sulfonic or phosphoric acid together with a crosslinking agent comprising a multi-valent metal ion capable of crosslinking an acrylic acid polymer to form a viscous gel. The injected fluid is flowed through at least a portion of a high permeability region within said zone wherein it is heated to an elevated temperature whereupon crosslinking of the polymers occurs to form a substantially non-flowable gel within said high permeability region. The crosslinking of the injected fluid to form the non-flowable gel within the formation reduces the permeability of said region in said zone.

The present invention relates to polymers and their preparation andtheir use in oil and gas fields.

Oil and gas fields produce water as well as oil and/or gas, especiallywhen the well is depleted. In addition, secondary recovery techniquessuch as water flooding to stimulate production of oil involve injectionof water under pressure at a distance from a production well to squeezethe oil out. However, in both cases the water moves in the formationalong least hindered paths, so that the recovery technique may beinefficient and in the direct recovery increased proportions of waterare produced.

To enhance reservoir conformance control, i.e. mobilise the oil that maybe present in less permeable areas, blocking agents may be injected toobstruct the high permeability channels thereby encouraging preferencefor liquid movement via the lower permeability channels. Among knownblocking agents are polymer gels, in particular, gels of polyacrylicacid or polyacrylamide/polyacrylic copolymers, cross-linked withchromium ions as disclosed in U.S. Pat. Nos. 4,744,418 and 4,844,168.The copolymer, mixed with cross-linker, is injected into the formationfrom the production well, often after a flood of cold water to pre-coolthe formation rock, to stop premature cross-linking and gelling beforethe mixture reaches its desired position. Much work has been describedto reduce the rate of cross-linking, by reducing the activity of thecross-linking metal ion, e.g. by co-ordinating the chromium with aligand, e.g. lactate.

An improved cross-linking system has now been discovered which comprisesa composition in which the tendency of the polymer to cross-linkprematurely has been reduced further by use of a copolymer of apolyacrylamide with a polar monomer.

The present invention provides a water soluble copolymer of:

(i) at least one non acidic ethylenically unsaturated polar monomer and

(ii) at least one copolymerisable ethylenically unsaturated ester.

The invention also comprises compositions comprising the water solublecopolymer and a polyvalent metal ion gelling agent.

The present invention also provides a process for delaying the rate ofgelling of an acrylic polymer with a gelling agent, in which the acrylicpolymer is said copolymer comprising (i) and (ii).

The invention also provides a method for conformance control of areservoir, especially one with high formation temperatures, which methodcomprises:

(a) injecting into the formation an aqueous solution of saidcomposition,

(b) allowing the solution to flow through at least one zone of highpermeability in said formation under increasing temperature conditions,

(c) allowing the composition to gel under said conditions.

The copolymer is formed from at least one, e.g. 1-3 polar monomer(s) andat least one, e.g. 1-3 ester(s), and comprises structural units derivedfrom said monomer(s) and ester(s); preferably the copolymer consistsessentially of said structural units. The ester is substantially neutralas it is a fully esterified derivative of an acid, i.e. complete ester,rather than a partial ester with free acid group(s).

The ester is usually formed from a hydroxyl compound and anethylenically unsaturated carboxylic acid. The ethylenically unsaturatedgroup may be in the alpha-beta or beta gamma position relative to thecarboxyl group or may be further distant; preferred acids have 3-20carbons, such as 3-12, e.g. alkenoic and aralkenoic acids with 3-6 or9-12 carbons respectively. Examples of the acids are acrylic,methacrylic, crotonic and cinnamic acids. The hydroxyl compound isusually an alcohol and may be of formula ROH, where R is a hydrocarbylgroup, preferably an alkyl group, e.g. of 1-30 or 2-30 such as 1-6, 2-6,7-30 or 7-24 carbons, alkenyl groups, e.g. of 2-20 carbons such as 2-6carbons, cycloalkyl group, e.g. of 5-8 carbons, aryl group, e.g.aromatic hydrocarbyl group such as 6-20 carbons or aralkyl group, e.g.of 7-24 carbons. Examples of such R groups are methyl, ethyl, n- and isopropyl, n, sec, iso and tert butyl, n, sec, iso and tert amyl and hexyl,octyl and 2-ethylhexyl, decyl, allyl, cyclohexyl, palmityl, steryl,phenyl and benzyl. The R group may also be a hydrocarbyl group,substituted by at least one substituent e.g. 1-3 substituents,especially from hydroxyl, ether, and thio ether groups; electrondonating groups are preferred. Ether substituents are preferred,especially alkoxy, aryloxy and aralkoxy, in which the alkyl, aryl andaralkyl groups may be as described above. Preferably the substituent ison the same carbon atom of the R group as is bonded to the hydroxylgroup in the hydroxyl compound; alkoxymethyl and aralkoxy methyl groupsare preferred. The hydroxyl compound may be a primary, secondary, iso ortertiary compound, especially with a tertiary carbon atom bonded to thehydroxyl group, e.g. tert butyl and trityl. The group R may alsocomprise a heterocyclic group either for bonding directly to thehydroxyl group of ROH or separated therefrom by an alkylene group, e.g.of 1-4 carbons such as methylene. Thus group R may be a saturated orunsaturated heterocyclic or heterocyclic alkylene group, e.g. of 3-8carbons and at least one, e.g. one or two ring heteroatoms selected from0, N and S, especially 0 and/or N, examples of such groups are furyl,tetrahydrofuryl, furfuryl and tetrahydrofurfuryl, pyranyl and tetrahydropyranyl. Most preferred R groups are tert-butyl, trityl,methoxymethyl, benzyloxymethyl and tetrahydropyranyl; stearyl, isopropyl, ethyl and methyl may also be preferred.

The ester (i) may also be derived from a hydroxyl compound, e.g. offormula ROH and an ethylenically unsaturated sulphonic or phosphoricacid, which may contain 2-20 carbons, especially 2-6 carbons, such asalkenyl acids, e.g. vinyl sulphonic acid and vinyl phosphonic acid. Thusthe ester may be methyl or ethyl vinyl sulphonate or phosphonate. Theester may be derived from an acid containing an ethylenicallyunsaturated carboxamide (e.g. acrylamido) group, as well as thesulphonic or phosphonic acid group; an example of such an acid is2-acrylamido-2-methylpropane sulphonic acid.

The ester is copolymerised with an ethylenically unsaturated polarmonomer, in which the unsaturated group is usually vinyl or alpha methylvinyl and may be derived from an unsaturated carboxylic acid (the acidbeing as further described above) e.g. primary, secondary or tertiaryamide thereof, in which the amide is derived from ammonia, or a primaryor secondary alkylamine, e.g. of 1-10 carbons, which may optionally besubstituted by at least one hydroxyl group as in alkylol amides such asethanolamides; examples of such carboxylic derived polar monomers areacrylamide, methacrylamide and acrylic ethanol amide. The polar monomermay also be a vinyl heterocyclic compound e.g. with at least one O, S orN atom in a ring with 3-8 carbons such as one with at least one carbonylgroup in the ring, e.g. N-vinyl-pyrrolidone or -caprolactam, or a vinylpyridine.

The copolymer may contain 0.01-50% e.g. 0.1-40% or 1-30%, especially5-15% mol of structural units from said ester(s) and 99.99-50% e.g.99.9-60% or 99-70 or 95-85% mol of structural units from said polarmonomer(s). The copolymer may be a block or non block copolymer, e.g. aregular or random copolymer or a graft copolymer, especially with esterunits grafted onto polymeric polar, monomer, e.g. ester grafted onpolyacrylamide.

The copolymer is usually soluble in water to an extent of at least 1 g/le.g. 1-200 g/l such as at least 10 g/l in distilled water at 15° C.,especially in aqueous sodium chloride solution containing 32 g/l NaCl at25° C. If desired, the copolymer may be mixed with a surfactant (e.g. inamount of 0.01-5% by wt of the solution) to help solubilise it in thewater or sodium chloride solution.

The copolymer may have a weight average molecular weight of at least50,000 e.g. 50,000-20 million, such as 100,000 to 10 million, especially100,000-500,000 or 1-10 million; the molecular weight may be determinedby conventional methods, e.g. gel permeation chromatography or intrinsicviscosity. The low mole wt copolymer may have a viscosity in aqueous3.6% wt solution at 19° C. of 10-500 cps (measured at 60 rpm with aHaake viscometer). Preferably the copolymer is sheer thinnable, e.g.with the viscosity reducing by at least 10% on increasing the sheer rateby 10%.

The copolymer may be made by conventional methods for copolymerisingethylenically unsaturated monomers in solution, emulsion or suspension,(preferably aqueous), such as are described in Encyclopaedia of PolymerScience & Engineering, Ed. Mark, Bikales, Overberger and Menges, Publ.Wiley Interscience, New York, 2nd Ed., Vol 1, pp 181-211 and referencescited therein, especially L. J. Young in J Brandrup and E. H. ImmergutEds Polymer Handbook, J Wiley, New York, 2nd Ed. 1975, Sec. II and 3rdEd. Sec. III, especially pp 155/6 and references cited therein and R ZGreenley J Macromol Science Chem. 14, 427, 445 (1980) and G Saini et alMakromol, Chem. 144, 235 (1971), the disclosure of each of which isincorporated herein by reference. Free radical aqueous suspension oremulsion polymerisation is preferred.

The composition of the invention comprises the copolymere and apolyvalent metal ion, capable of crosslinking an acrylic acid polymer inaqueous solution to form a gel. The cross-linking may be at 20°-200° C.,especially 50°-150° C. The metal ion is usually 2, 3 or 4 valent and maybe of group 2A, 3A, 4A, 5A, 6A, 7A, 8, 2B, 3B or 4B of the PeriodicTable, e.g. Ca, Mg, Ba, Ti, Zr, Fe, Zn, Al or Sn; preferably the metalion is 3 or 4 valent and is especially a transition metal such aschromium or iron, though aluminium may also be used. The particularlypreferred metals are chromium and zirconium. The weight ratio of themetal to copolymer is usually 1:1-100 such as 1:10-80, especially1:25-50. The ratio of the number of g atoms of metal to g moles ofcopolymer is usually 1000:0.001-100, preferably 1000:0.01-10, such as1000:0.1-20 or 1-10, especially for copolymers of Molec. Wt100,000-500,000. The ratio of the number of g atoms of metal toequivalents of ester group in the copolymer is usually 1000:0.5-5000,preferably 1000:5-5000 such as 1000:50-1000 or 1000:500-5000. The metalis usually present as a salt, e.g. an inorganic salt such as a halideespecially chloride, nitrate or sulphate or as a carboxylateparticularly a monodentate carboxylate, such as a hydrocarbylmonocarboxylate, e.g. with 1-24 carbon atoms (e.g. 2-6) in thecarboxylic acid, which may be an alkanoic acid such as acetate. Themetal may also be complexed, e.g. with a ligand, such as a carboxylicacid group, having at least 2, e.g. 2-4 dentate groups in particular, asdescribed in Canadian Patent 2063877, the disclosure of which is hereinincorporated by reference; example of complexing carboxylic acids arehydroxy alkanoic acids, e.g. lactate, glycollate or tartrate. The saltor complex is water soluble. The composition is usually made just beforeuse by adding the copolymer and the metal ion (e.g. as salt or complex)to an aqueous medium, e.g. sea water and then injecting the aqueouscomposition made into the formation. The composition is preferably keptat below 50° C., e.g. below 30° C. before use. The concentration ofcoplymer in the aqueous composition may be 500-100,000 ppm, inparticular 500-10,000 ppm for copolymers of high Molecular wt of atleast 1 million and 10,000-50,000 ppm for copolymers of lower Molecularwt. 50,000-1 million. The concentration of the cross-linking metal ionin the aqueous composition may be 10-3000 ppm, especially 10-250 ppm and3000-1500 ppm respectively for said high and low Molec wt copolymers.

The aqueous composition may be injected into the formation via aproducing well or via a secondary injection well (for use with awaterflood or squeeze technique). The aqueous composition may alsocontain other ingredients, e.g. antioxidants and/or oxygen scavengers.The injection may, if desired, be preceded by a precooling treatment,e.g. coldwater to stop premature cross-linking but preferably theinjection process is performed in its absence. The aqueous compositionmay simply be injected with the formation but preferably is forced intoit by pumping. The formation is usually at 50°-180° C., especially60°-100° C. or 100°-150° C. and is generally water bearing rather thanoil bearing. It may be of acidic rock, e.g. sandstone or neutral tobasic rock, e.g. limestone with associated formation water of e.g. pH3-6.5 such as 4-6 or pH 6.5-8 respectively. The compositions of theinvention are especially suitable for use with acidic rocks, especiallyat 60°-150° C.

In particular, copolymers with carboxylic esters from tertiary alkanolsor arylmethanols or from ether substituted alkanols or heterocyclicalcohols may be used with acidic rocks at 60°-100° C. and esters fromother hydroxy compounds e.g. primary or secondary alkanols at 100°-150°C. The well may be shut in for 1-70 hr to allow the gelling to occur andthen production may be restarted.

The copolymers and compositions of the invention have a benefit of areduced tendency to cross-linking and gelling in the well bore (i.e.reduced aggregate build up) but rapid cross-linking at the hightemperatures of the formation. They therefore are less susceptible toprocess handing problems.

The invention is illustrated in the following Examples:

EXAMPLE 1

A 90:10 molar copolymer of acrylamide and tertiarybutyl acrylate inaqueous solution was made by free radical copolymerisation, according tothe general technique of the references mentioned herein.

The copolymer had a viscosity of 55 cps in 3.6% solution in sea water at19° C. (the viscosity being determined as described above). By gelpermeation chromatography (GPC) its weight (average molecular weight)was about 295,000, the number average Molec wt was 44,900 and the peakMolec wt was 229,000.

The copolymer solution and a solution of chromium triacetate were addedto API sea water (3.2% total dissolved solids) at 25° C. to give anaqueous injection solution containing 36,000 ppm copolymer and 1000 ppmCr ion.

EXAMPLE 2

The process of Example 1 was repeated to make a 90:10 molarcopolymer ofacrylamide and methyl acrylate and an injection solution therefrom. Thecopolymer had a viscosity of 50 cps in 3.6% solution in sea water at 19°C. (the viscosity being determined as described above). The Molec wtdate (by GPC) were MW 293,000, Mn 42,600 and Peak Mo. wt. 231,000.

EXAMPLES 3 and 4

The polymerisation processes of Examples 1 and 2 are repeated to givecopolymers with weight average molecular weight of 5-10 million. Eachcopolymer and chromium acetate are added to sea water to give aninjection solution containing 3000 ppm concentration of copolymer and a75 ppm concentration of Cr.

EXAMPLE 5

A 10 ft stainless steel tube of internal diameter 1/4" was packed withquartz sand of average particle size of 90 u. API sea water (3.2% TotalDissolved Solids) was pumped into the tube at 80° C. until a constantdifferential pressure was obtained. The absolute permeability of thesand pack was calculated by means of Darcy's law to be 1206MilliDarcies. The tube was then flooded with Forties Crude Oilcontaining 15% toluene followed by another flush of API sea water toconstant differential pressure. The permeability of the sand pack atresidual oil saturation was calculated to be 280 MilliDarcies.

The aqueous injection solution of Example 1 was pumped into the tube(maintained at 80° C. by means of an oven) at a flow rate of 10 mls/hrand in such a way as to maintain a retention time of 1/2 hour in theinlet tube at 80° C. prior to entry into the sand pack. The maximumpressure limit was set at 100 bar and this limit was reached afterapproximately four hours, after which the injection solution wasautomatically injected at a progressively lower flow rate. The injectionsolution was left to "squeeze" into the tube for approximately a further16 hours by which time no further flow could be detected. A total of 55mls of the solution had flowed into the sand pack. In order to ascertainthe depth of the gel block the tube was placed under reverse flow whilemaintaining a differential pressure of 100 bar. During this operation,five inch sections of tube were cut from the outlet end (previously theinlet end) until the flow could be detected. The tube was thus found tobe blocked to at least 9 feet.

EXAMPLE 6

A 10 foot tube, as in Example 5, was found to have an absolutepermeability of 117_(m) D and a permeability of 263_(m) D at residualoil saturation.

The injection solution of Example 2 was used in the gellation test ofExample 5 under identical conditions to Example 5, except that the oventemperature was at 110° C. A total of 58 mls of injection of solutionhad been pumped prior to blocking and the tube was found to be blockedin a depth of 9.5 feet.

EXAMPLES 7 and 8

The processes of Examples 5 and 6 are repeated with the injectionsolutions of Examples 3 and 4. Gellation caused blocking.

I claim:
 1. A method for conformance control of a reservoircomprising:injecting into a zone of the reservoir an aqueous solution ofa co-polar comprising at least one ethylenically unsaturated polarmonomer and at least one copolymerizable ethylenically unsaturated esterformed from a hydroxy compound of the formula ROH wherein R is an alkylgroup of 4-30 carbons, alkenyl group of 4-20 carbons, cycloalkyl groupof 5-8 carbons, aryl group of 6-20 carbons or such groups, substitutedwith from 1 to 3 hydroxyl, ether or thio ether groups or a heterocyclicor heterocyclic alkylene group of from 3-8 carbons and at least oneheteroatom selected from oxygen, nitrogen and sulfur and an alkenoic oraralkenoic carboxylic acid having from 4-20 carbons or sulfonic orphosphoric acid having from 2-20 carbons together with a crosslinkingagent comprising a multi-valent metal ion capable of crosslinking anacrylic acid polymer to form a viscous gel; allowing the solution toflow through at least a portion of a high permeability region withinsaid zone wherein it is heated to a temperature of at least about 20° C.while maintained at a pH above about 6.5 whereupon crosslinking occursto form a substantially non-flowable gel within said high permeabilityregion which reduces the permeability of said region in said zone. 2.The method of claim 1 wherein said solution is heated to a temperatureabove about 90° C. in said region of said reservoir.
 3. The method ofclaim 1 wherein R comprises at least one member selected from the groupsof N, sec, iso or tertiary butyl, n, sec, iso or tertiary amyl, n, sec,iso or tertiary hexyl, octyl, 2-ethylhexyl, decyl, allyl, cyclohexyl,palmityl, stearyl, phenyl, benzyl, furyl, tetrahydrofuryl, furfuryl,tetrahydrofurfuryl pyranyl and tetrahydropyranyl.
 4. The method of claim1 wherein R comprises at least one member selected from the group oftertiary butyl, trityl, benzyloxymethyl, stearyl and tetrahydropyranyl.5. The method of claim 1 wherein the multi-valent ion os saidcrosslinking agent comprises at least one member selected from chromium,iron, aluminum and zirconium.
 6. The method of claim 1 wherein saidsolution is heated to a temperature in the range of from about 90°-150°C. in said high permeability region.
 7. A method of blocking a highpermeability region in a zone of a subterranean formation penetrated bya wellbore comprising:injecting into a zone of a subterranean formationhaving a temperature of from about 60°-150° C. an aqueous solution of acopolymer comprising at least one ethylenically unsaturated polarmonomer and at least one copolymerizable ethylenically unsaturated esterformed from a hydroxy compound of the formula ROH wherein R is an alkylgroup of 4-30 carbons, alkenyl group of 4-20 carbons, cycloalkyl groupof 5-8 carbons, aryl group of 6-20 carbons or such groups, substitutedwith from 1 to 3 hydroxyl, ether or thio ether groups or a heterocyclicor heterocyclic alkylene group of from 3-8 carbons and at least oneheteroatom selected from oxygen, nitrogen and sulfur and an alkenoic oraralkenoic carboxylic acid having from 4-20 carbons or sulfonic orphosphoric acid having from 2-20 carbons together with a crosslinkingagent comprising a multi-valent metal ion capable of crosslinking anacrylic acid polymer to form a viscous gel; shutting-in said wellborefor from about 1 to about 70 hours.
 8. The method of claim 7 whereinsaid formation is sandstone having a formation water pH of from about3-6.5.
 9. The method of claim 7 wherein said formation is limestonehaving a formation water pH of from about 6.5-8.
 10. The method of claim7 wherein R comprises at least one member selected from the groups of N,sec, iso or tertiary butyl, n, sec, iso or tertiary amyl, n, sec, iso ortertiary hexyl, octyl, 2-ethylhexyl, decyl, allyl, cyclohexyl, palmityl,stearyl, phenyl, benzyl, furyl, tetrahydrofuryl, furfuryl,tetrahydrofurfuryl pyranyl and tetrahydropyranyl.
 11. The method ofclaim 7 wherein R comprises at least one member selected from the groupof tertiary butyl, trityl, benzyloxymethyl, stearyl andtetrahydropyranyl.
 12. The method of claim 7 wherein the multi-valention os said crosslinking agent comprises at least one member selectedfrom chromium, iron, aluminum and zirconium.
 13. A method for reducingthe permeability of a relatively higher permeability zone of asubterranean sandstone formation penetrated by a wellborecomprisingintroducing into the relatively higher permeability zone ofsaid formation through said wellbore a quantity of an aqueous solutionof a copolymer comprising at least one copolymerizable ethylenicallyunsaturated ester formed from a hydroxy compound of the formula ROHwherein R is an alkyl group of 4-30 carbons, alkenyl group of 4-20carbons, cycloalkyl group of 5-8 carbons, aryl group of 6-20 carbons orsuch groups, substituted with from 1 to 3 hydroxyl, ether or thio ethergroups or a heterocyclic or heterocyclic alkylene group of from 3-8carbons and at least one heteroatom selected from oxygen, nitrogen andsulfur and an alkenoic or aralkenoic carboxylic acid having from 4-20carbons or sulfonic or phosphoric acid having from 2-20 carbons togetherwith a crosslinking agent comprising a multi-valent metal ion capable ofcrosslinking an acrylic acid polymer to form a viscous gel; and heatingsaid aqueous solution to a temperature in excess of about 90° C. withinsaid formation while subjecting said solution to an acidic pH of belowabout 6.5 whereupon a crosslinking reaction occurs to form asubstantially non-flowable gel in said formation to substantiallynon-flowable gel in said formation to substantially decrease thepermeability of the portion of said formation occupied by said gel. 14.The method of claim 13 wherein said solution is heated to a temperatureabove about 90° C. in said region of said reservoir.
 15. The method ofclaim 13 wherein R comprises at least one member selected from thegroups of N, sec, iso or tertiary butyl, n, sec, iso or tertiary amyl,n, sec, iso or tertiary hexyl, octyl, 2-ethylhexyl, decyl, allyl,cyclohexyl, palmityl, stearyl, phenyl, benzyl, furyl, tetrahydrofuryl,furfuryl, tetrahydrofurfuryl pyranyl and tetrahydropyranyl.
 16. Themethod of claim 13 wherein R comprises at least one member selected fromthe group of tertiary butyl, trityl, benzyloxymethyl, stearyl andtetrahydropyranyl.
 17. The method of claim 13 wherein the multivalention os said crosslinking agent comprises at least one member selectedfrom chromium, iron, aluminum and zirconium.
 18. The method of claim 13wherein said solution is heated to a temperature in the range of fromabout 90°-150° C. in said high permeability region.